Running woe #1 Sticking high idle
Right after I got the bike running, I struggled with a strange engine issue.
When I'd fire the bike up, it would idle at a sane RPM, but when I revved it up, the idle wouldn't come back down on its own. I would be able to pull the idle down to normal levels by leaving the bike in gear as I braked, letting the brakes lug the engine back down to a sane idle speed. I would also be able to get the idle down to a sane level as I rolled along by shutting off the engine with the kill switch, then switching it back on so the bike was push started back to life, idling happily at a reasonable RPM.
I didn't believe that the idle was simply set too high because it was possible to bring the idle back to a sane level by restarting the bike or lugging it back down.
However, simply turning down the idle made this problem disappear. I'm not sure if the fact that I could pull down the idle from its high setting is typical of CV-type carburetors, or if it was because the bike was running lean (see future post), or because the slide return springs were much too long (see future post).
Unfortunately, even after I turned down the idle, when I'd rev the engine up, the RPMs would hover at high RPMs for a while before dropping back down to a more sane level on their own. Also, when idling at a light, the engine RPMs would steadily drop and the engine would eventually stall out after about 10 seconds if I didn't occasionally blip the throttle. On the other hand, turning down the idle made the bike much more more driveable.
When I'd fire the bike up, it would idle at a sane RPM, but when I revved it up, the idle wouldn't come back down on its own. I would be able to pull the idle down to normal levels by leaving the bike in gear as I braked, letting the brakes lug the engine back down to a sane idle speed. I would also be able to get the idle down to a sane level as I rolled along by shutting off the engine with the kill switch, then switching it back on so the bike was push started back to life, idling happily at a reasonable RPM.
I didn't believe that the idle was simply set too high because it was possible to bring the idle back to a sane level by restarting the bike or lugging it back down.
However, simply turning down the idle made this problem disappear. I'm not sure if the fact that I could pull down the idle from its high setting is typical of CV-type carburetors, or if it was because the bike was running lean (see future post), or because the slide return springs were much too long (see future post).
Unfortunately, even after I turned down the idle, when I'd rev the engine up, the RPMs would hover at high RPMs for a while before dropping back down to a more sane level on their own. Also, when idling at a light, the engine RPMs would steadily drop and the engine would eventually stall out after about 10 seconds if I didn't occasionally blip the throttle. On the other hand, turning down the idle made the bike much more more driveable.
posted by veloandy at 12:40 PM
2 Comments:
This run-away idle problem is making me NUTS on my XS400. Have you made any advances with this problem? I too am doing exactly what you have done. I could have written your post myself.
It's consistant on these bikes. It isn't just yours. If thats any consolation at all :)
-Corky in Hollywood Fl.
Hey Corky,
I did figure this out. I think these bikes were set up to run VERY lean from the factory. Three decades of crud collecting in the pilot jets doesn't help either.
To fix it, I:
1. Made sure there weren't any vacuum leaks
2. replaced the pilot jets
3. Actually set the richness correctly with a ColorTune (described in detail here)
It was step 3 that REALLY fixed everything.
Now it runs *perfectly*!
Good luck!
-Andy
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